Paliperidone, chemically named as (±)-3-[2-[4-(6-Fluoro-1,2-benzisoxazol-3-yl)-1-piperidi-nyl]ethyl]-6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H-pyrido pyrimidin-4-one has a structure of formula I:

Paliperidone is psychotropic agent approved in the United States under the trade name Invega®.
Palmitate ester of Paliperidone, chemically named as (9RS)-3-[2-[4-(6-Fluoro-1,2-benzisoxazol-3-yl)piperidinyl]ethyl]-2-methyl-4-oxo-6,7,8,9-tetrahydro-4H-pyrido-[1,2-a]-pyrimidin-9-yl hexadecanoate has a chemical structure of formula II:

Paliperidone palmitate is psychotropic agent and marketed in the United States under the trade name Invega Sustenna®. Paliperidone palmitate is indicated for the treatment of schizophrenia and is available in the dosage form of extended release injectable suspension for intramuscular administration.
Paliperidone and Paliperidone palmitate were described for the first time in the U.S. Pat. No. 5,158,952. The U.S. '952 discloses processes for the preparation of Paliperidone as well as decanoyl and acetyl esters of Paliperidone.
Various processes for preparing Paliperidone Palmitate are described in U.S. Pat. No. 6,077,843, U.S. Pat. No. 6,555,544, WO 2009/089076, WO 2012/164582, WO 2013/046225, IN 2383/CHE/2010, IN 3486/CHE/2012, IN 1685/MUM/2012, IN 113/MUM/2012, IN 2252/MUM/2011 and IN 3372/MUM/2012.
Like any other synthetic APIs, Paliperidone palmitate may contain extraneous compounds or impurities arising from the unreacted starting materials, by-products and/or degradation products. Impurities in API are undesirable and, in extreme cases, might even be harmful to a patient being treated with a dosage form of the API.
Further, solid state physical properties such as particle size of an active pharmaceutical ingredient (API) are very important in formulating a drug substance and can have profound effects on the ease and reproducibility of formulation. Small size of particles of Paliperidone Palmitate API is desired for the production of its extended-release injectable suspension.
The PCT application WO 2006/114384 discloses a process for preparing aseptic crystals of Paliperidone Palmitate having suitable particle size for using in the pharmaceutical composition. WO '384 discloses a process, which comprises steps of dissolving non-sterile Paliperidone Palmitate in ethyl alcohol, filtering the solution through sterile 0.22 μm filter, cooling the solution to room temperature to crystallize the product, reheating the suspension and cooling at a rate of 0.5° C./min to obtain aseptic crystals of Paliperidone palmitate.
However, this process is not suitable for industrial production of Paliperidone palmitate API as it involves repeated crystallization steps and lengthy process of cooling at slow rate to crystallize the product. Further, Paliperidone palmitate obtained by single crystallization from ethyl alcohol does not yield the Paliperidone palmitate of suitable particle size for using in extended-release injectable suspension.
Hence, there is a need in the art to develop a process for preparing aseptic Paliperidone Palmitate having a particle size suitable for use in the pharmaceutical composition.